I think this is a great thread. I have been thinking about this too.
As much as I love Suzie, and do think she has a great voice for the band, I don't think she is a leader.
For me, it's between Marty and JD....for exactly the reasons that you indicate. JD could carry on where the band left off. He is fresh enough and young enough to pick up where Michael was starting to fade, but he could do the job. Marty is different and could take the band in a new direction while doing justice to the back catalogue. I think that when we see who INXS chooses, we'll know the direction that the want to go in.
I totally disagree with the post that they are washed up. Their body of work is amazing. I think a new spin on their songs could work.